1. Field of the Invention
The present invention relates to a semiconductor laser device capable of stable operation at the time of high power output and a method of manufacturing the same, and an optical transmission module and an optical disk apparatus using the semiconductor laser device.
2. Description of the Background Art
In semiconductor laser devices widely used as light sources for optical disk apparatuses, optical communication apparatuses or the like, higher power output of the semiconductor laser devices is in increasing demand with increasing demand for higher-speed operation of optical disk apparatuses, optical communication apparatuses or the like. In achieving higher power output of a semiconductor laser device, COD (Catastrophic Optical Damage) degradation at a resonator end surface as a laser light emitting surface is a major concern and gives a significant effect on long-term reliability of high power operation. This COD degradation is thought to be caused by the following mechanism. More specifically, since a large number of surface states are present at the resonator end surface due to an impurity or the like, carriers are absorbed through these states to cause non-radiative recombination current to flow, thereby locally increasing the temperature. This temperature increase reduces the bandgap in the vicinity of the resonator end surface and, in addition, increases absorption of laser light and thus increases the end surface temperature. Repetition of such positive feedback causes melting of the resonator end surface and stops oscillation.
As one of the approaches to solve this problem, Japanese Patent Laying-Open No. 10-209562 discloses a semiconductor laser device and a method of manufacturing the same as follows. A semiconductor laser device shown in FIG. 24 is fabricated in the following manner. First, an n-GaAs buffer layer 502, an n-AlGaAs lower cladding layer 503, an AlGaAs lower guide layer 504, a quantum well active layer 505 formed by alternately stacking an InGaAs well layer (two layers) and a GaAs barrier layer (three layers), an AlGaAs upper guide layer 506, a p-AlGaAs upper cladding layer 507, a p-GaAs cap layer 508 are successively stacked on an n-GaAs substrate 501 by a metal-organic chemical vapor deposition (MOCVD) method.
Then, a stripe mask (not shown) formed of SiO2 is fabricated by usual photolithography step and etching step. Using the stipe mask, p-GaAs cap layer 508 and p-AlGaAs upper cladding layer 507 are partially etched to form a ridge-like stripe portion.
Then, buried crystal growth is successively performed on a side portion of the stripe portion with an n-AlGaAs current block layer 509 and an n-GaAs current block layer 510 by a selective growth technique using the stripe mask. Then, after removal of the stripe mask, a p-GaAs cap layer 511 is crystal-grown. Finally, a p-side electrode (not shown) is deposited on the upper surface of p-GaAs cap layer 511 and an n-side electrode (not shown) is deposited on the back surface (the opposite surface to the surface on which each of the aforementioned semiconductor layers is stacked) of n-GaAs substrate 501.
The resulting stacked structure is cleaved as a whole in a bar shape with a width of the resonator length, and a low reflection film and a high reflection film (not shown) are respectively grown on the two exposed cleaved surfaces. The bar is then divided into chips, thereby completing a semiconductor laser device.
In this method of manufacturing a semiconductor laser device, cleavage into a bar shape as described above is performed in ultrahigh vacuum. This is because production of an impurity layer such as an oxide film at the resonator end surface exposed by the cleavage can effectively restrained as compared with cleavage in the atmosphere. In this case, light absorption by the interface state of the impurity layer at the resonator end surface can be reduced, so that the COD degradation at the end surface can be restrained. It is therefore thought that a semiconductor laser capable of stable operation at the time of high power output can be obtained.
Furthermore, Japanese Patent Laying-Open No. 10-084162 discloses a method of manufacturing a semiconductor laser device including a step of removing an impurity at a resonator end surface. In Japanese Patent Laying-Open No. 10-084162, first, as shown in FIGS. 25, 26, a semiconductor wafer 530 having crystal growth and electrode formation is cleaved, resulting in a laser bar 532 having a resonator end surface 531. Then, as shown in FIG. 27, the above-noted laser bar 532 is put into a vacuum container 534 with a plasma generation apparatus 533, and resonator end surface 531 exposed by the cleavage is irradiated with Ar plasma 535. Thereafter, a reflection film is formed on the resonator end surface in vacuum container 534. As a result, moisture attached on resonator end surface 531 is removed, so that defects at the end surface resulting from moisture can be reduced. It is therefore thought that a semiconductor laser device capable of high power operation with improved COD level can be obtained.
However, in the method of manufacturing a semiconductor laser device as disclosed in Japanese Patent Laying-Open No. 10-209562, since cleavage into a bar shape needs to be performed in ultrahigh vacuum, the manufacturing apparatus is inevitably complicated and in addition, the yield is reduced. On the other hand, in the method of manufacturing a semiconductor laser device as disclosed in Japanese Patent Laying-Open No. 10-084162, moisture on the resonator end surface obtained by the cleavage can be removed. However, the semiconductor crystal is damaged by plasma radiation at the resonator end surface, and the interface state resulting from the damage is produced. Therefore, the improvement of COD level by reducing defects at the resonator end surface is not necessarily achieved, and a stable high power operation is not necessarily achieved.